This research is directed towards an understanding and clarification of basic visual-perceptual processes. Specifically it is directed toward extending our knowledge of the nature of perception during pursuit eye movements (pems). The work entails an investigation of a set of phenomena associated with pems which suggest that there is a loss of both position and direction constancy, i.e., stationary objects which are viewed during an interval in which a moving object is pursued by the eyes appear to move or change their positions. If in fact there is a total loss of constancy during pems, it suggests either that perception during pems is unlike other modes of perception which tend to be characterized by constancy or that the well established view that perception is characterized by constancy is in need of revision. To be clearer: constancy perception is perception which remains faithful to the objects in the world rather than to various changes in retinal stimulation occasioned either by our or the object's movements. (e.g., we tend to perceive the size of a given object as constant despite changes in its retinal image size caused by changes in the object's distance from us). A theoretical account of the various instances of apparent loss of constancy associated with pems is proposed which interprets these instances within a constancy framework. An attempt will be made to determine the legitimacy of this explanation along with the attempt to increase our understanding of the quality of perception during pems. Human subjects will be tested.